Because of the advantages of large capacity, high transmission efficiency and low electromagnetic radiation to environment, the DC transmission technology has been paid attention in the modern energy transmission of new generation. Due to the DC transmission, the current on the transmission line cannot be measured by the mutual induction device utilizing electromagnetic induction. How to measure the DC current becomes a technical problem. In the prior art, Hall effect is used for measuring the DC current, however this method has the defects of lower sensitivity, higher equipment cost and difficult high-voltage insulation.
Currently magneto-optic effects such as Faraday Effect are widely used in measurement of the electrical current and magnetic field. Magneto-optic glass, magneto-optic crystal or fiber is usually used for achieving Faraday Effect. Since the optical intensity of the carrier of the magneto-optic effect is a DC signal, it is difficult to maintain the signal unchanged in the life of several years, even more than ten years. If the DC magnetic field or current is measured using the magneto-optic effect, the problem will be raised since it is difficult to separate the electric current signal from the optical intensity signal.
In FIG. 1, a system for fiber magneto-optic detection in the prior art is shown comprising a power supply and signal processing module 400, a magneto-optic probe 100 and a conductive fiber device 200. The power supply and signal processing module 400 emits laser light which is transmitted to the magneto-optic probe 100 through the fiber device 200. The magneto-optic probe is designed to sense the magnetic field of the environment in which the magneto-optic probe 100 is located according to a certain optical principle such as Faraday Effect, and convert the magnetic field signal into optical signal. The optical signal is carried by the laser light and transmitted back to the power supply and signal processing module 400 via the fiber device 200. The power supply and signal processing module 400 performs data processing on the optical signal received, so as to obtain information at the relevant magnetic field at the point measured.
The measurement process shown in FIG. 1 may be described by the following equation:I=I0(1+c1H)  (1)
where I represents the optical signal detected by an photoelectric detection device in the power supply and signal processing module 400; I0 represents a parameter relating to the optical intensity of the emitted laser light and the transmission ratio; c1 represents a magneto-optical parameter relating to the characteristics of the magneto-optical material and to the polarization detection mechanism of the magneto-optic probe 100; and H represents the magnetic field to be measured. In general, I0 and c1 may be affected by various environmental factors. If the magnetic field H to be measured is in the state of alternating current (AC), c1H may be obtained by respectively measuring the DC component I0 and the AC component I0c1H and performing a division calculation. If c1 does not vary following the temperature variation, the measured result may well represents the magnetic field to be measured, otherwise the influence to c1 caused by the environmental factors needs to be eliminated. If the magnetic field H to be measured is in the state of DC, I0 may not be removed when using the above method, causing the measurement result improper.
The inventor recognizes in developing the invention that various materials used in the magneto-optic measurement are always affected by the environmental factors such as temperature, stress or the intensity of the magnetic field to be measured, which will cause improper measurement. Especially, the inventor recognizes that when some material such as garnet crystal is used as a magneto-optic medium to measure the magnetic field, the microcosmic structure of magnetic domains inside the garnet crystal changes unpredictably when the crystal is subjected to a certain intensity of direct-current or alternating magnetic field, which affects the optical characteristics and leads to uncertain measurement results. In the art, there is no reasonable solution for resolving such problems.